The vaccine will be delivered over 3 years and be enough to vaccinate 25 million people at two doses each, the United Kingdom–based drug company said in a news release.

The announcement marks the first tangible step toward the creation of a world stockpile of H5N1 vaccines, a goal endorsed by WHO member countries at the World Health Assembly in May.

Following GSK's announcement today, US-based Baxter International and the French vaccine maker Sanofi Pasteur both announced their intention to donate H5N1 vaccines to the WHO stockpile, but neither company specified how many doses it would contribute.

The idea of a global stockpile emerged after complaints by Indonesia and other developing countries about lack of access to commercial H5N1 vaccines. Indonesia, the country hit hardest by H5N1, withheld samples of the virus from the WHO from last December until May on grounds that drug companies use the samples to make vaccines priced out of Indonesia's reach.

The WHO welcomed GSK's announcement today. "This is another significant step towards creating a global resource to help the world and especially to help develping countries in case of a major outbreak of H5N1 avian influenza," WHO Director-General Margaret Chan said in a news release.

The WHO said it needs to do "detailed operational planning for the stockpile, including how and under which conditions it will be deployed, as well as regulatory aspects of the vaccine."

After an international meeting of vaccine producers and government officials in late April, the WHO said it appeared that a global H5N1 vaccine stockpile might be feasible. Later, at the WHO's annual meeting in May, members passed a resolution calling for the creation of a stockpile of vaccines for H5N1 and other viruses of pandemic potential. The resolution also called on the agency to set up mechanisms for the "fair and equitable distribution" of pandemic flu vaccines at "affordable prices."

Dr. John Oxford, a virologist at London's Queen Mary School of Medicine, called GSK's announcement "excellent news," according to the Associated Press. "It's just what we need to reassure countries like Indonesia that they will get something in return from the viruses they provide, which will form the basis of these vaccines," he said.

GSK's vaccine includes a proprietary adjuvant (immune-stimulating chemical) and in clinical trials has induced a strong immune response at low doses, according to previous company announcements. Last July the company said 80% of volunteers showed a good immune response after receiving two 3.8-microgram doses. A typical dose of seasonal flu vaccine contains 15 micrograms of antigen for each of three flu strains.

In March GSK said its vaccine might protect people against more than one strain of H5N1. The vaccine is based on a 2004 strain from Vietnam, but in a clinical trial it elicited an immune response (neutralizing antibodies) to an H5N1 strain from Indonesia. However, the company has not yet published a detailed report of its clinical trial results.

The WHO noted that three other vaccine producers—Baxter, Sanofi Pasteur, and Omnivest of Hungary—also have expressed a willingness to provide some doses of H5N1 vaccine to the global stockpile.

Baxter's announcement today said the company intends to provide a "multiyear donation" of its candidate pandemic flu vaccine to the WHO. Sanofi Pasteur, meanwhile, said it was ready to give a "significant number of doses" of H5N1 vaccine, some of which could be supplied immediately in bulk form.

GSK promised that, in addition to its donation, it would provide some of its H5N1 vaccine to the WHO at "preferential" prices for use by countries eligible for assistance from GAVI (the Global Alliance for Vaccines and Immunization), a public-private program that provides vaccines for children in poor countries.

The company noted that a precisely matched vaccine won't become available until 4 to 6 months after a pandemic strain emerges. Experts hope that if the next pandemic strain is an H5N1 variant, existing H5N1 vaccines will provide some protection, though supplies are likely to be very short.